The present invention relates generally to a communication device; and, more particularly, it relates to a method for efficiently performing Header Check Sequence (HCS) operations in a cable modem having programmable MAC functionality.
In recent years, cable television networks have become widespread. A typical cable television system can carry many television stations, and is effectively a high bandwidth system. Because of the increasing availability of cable television infrastructure, the use of television cables as the medium for computer data networks has the potential for giving users high bandwidth at a reasonable cost. A cable television system, however, requires several enhancements in order to function as a data network.
In its classic form, a cable television system carries information in only one directionxe2x80x94from the cable system headend to the individual user. The user interface to the system generally comprises a receiver such as a television or a stereo. The headend transmits television or stereo channels simultaneously. In general, the user has no influence on what is transmitted and can only choose among the channels the headend is transmitting.
In contrast, a data network carries data from the headend to the user (the downstream path) and from the user to the headend (the upstream path). The individual user requires equipment, such as a cable modem, that can both receive from the headend and transmit to it. A cable data network must be able to handle many individual users simultaneously, each of whom have control over what they receive and transmit.
Cable modems offer greatly improved bandwidth capable of delivering services hundreds, or even thousands, of times faster than conventional modems. Cable modems can achieve data-transfer rates of up to 40 Mbits/s by connecting directly to coaxial lines as opposed to dial-in modems that use twisted-pair copper telephone lines.
In order for a cable television network to operate as a data network, it requires a headend capable of both transmitting and receiving data. To ensure that each user receives the data they require, a network protocol must be implemented to allow independent users of the network to utilize the shared headend and the distribution network without interference from or receiving the data of other users.
The network protocol places requirements on both the headend and the user end. Generally, the headend serves as the network controller, and the user""s cable modem must be able to respond to commands from the headend. In cable modems adhering to the well-known OSI reference model, the lowest layer is the Physical layer (PHY), while the next layer up is the Data Link layer. The Data Link layer is segmented into two parts, the Medium Access Controller (MAC), which interfaces with the PHY, and the Logical Link Control (LLC), which interfaces to the MAC and to higher layers. In general, the MAC and LLC provide the following Data Link functionality: transmit and receive data encapsulation, including framing (frame boundary delineation, frame synchronization), addressing (management of source and destination address), and error detection (detection of physical medium transmission errors); and media access management, including collision avoidance and handling. A physical address or MAC address is a unique Data Link layer address that is assigned to every port or device that connects to a network. Other devices in the network use these addresses to locate specific ports in the network and to create and update routing tables and data structures.
In an effort to coordinate the development of multimedia high-speed data services and the interoperability of network devices, cable operators have formed the Multimedia Cable Network Systems (MCNS) Group in cooperation with the industry research and development consortium CableLabs. The MCNS group has promulgated the Data Over Cable Service Interface Specification (DOCSIS). Other standards utilizing transport frames, such as DAVIC/DVB, have likewise been created. Such standards continue to evolve over time, with the frequent inclusion of additional feature sets. In specifications such as DOCSIS, MAC-layer frames are encapsulated in transport-layer frames, such as MPEG frames.
The term xe2x80x9ccable modem termination systemxe2x80x9d (CMTS) generally refers to a cable bridge or cable router in the cable head-end. A CMTS acts as the master station in a DOCSIS-compliant cable data system. The CMTS is generally the only station that transmits downstream, and it controls the scheduling of upstream transmissions by associated cable modems.
Cyclic-redundancy-checks (CRC) computations are generally performed on data received by applications supported by cable modems. For example, on the transmitting side, the CMTS typically computes a CRC value for frame headers. The CRC may be appended to the end of the header of each frame prior to transmission. On the receiving end, the cable modem extracts the header data and a CRC value is computed. This computed CRC value is then compared to the received CRC value appended to the header. If the values match, the header is assumed to be a valid header free of transmission errors. Even though a relatively small amount of data (e.g., 6-8 bytes) is involved, such CRC operations are very bit intensive and computationally difficult to perform in software.
Previously, cable modem devices have only included a fixed-function MAC in which a hardware state machine performs all functions on data as it arrives, including CRC operations. These devices are generally compliant with a single specification or a version of a specification. Thus, any changes to the underlying specification require concomitant hardware modifications, resulting in lengthy and expensive product development cycles.
Briefly, the present invention relates to a communication device, such as a cable modem, having a programmable media access controller (MAC) supported by a programmable CRC engine. The CRC engine computes CRC values for data written to it by the programmable MAC, thereby relieving the MAC processing circuitry of these bit-intensive computations. Use of the CRC engine thus improves the overall performance of a cable modem incorporating programmable MAC or similar functionality. The programmable nature of the CRC engine further permits compliance with a wide variety of standards, including evolving standards such as DOCSIS, without requiring expensive hardware upgrades.
In one embodiment of the invention, the CRC engine may be initialized by the programmable MAC with an initial vector prior to CRC computations. The programmable MAC or other software then locates the data bytes for which a CRC needs to be computed and forwards the information to the CRC engine. Based on the results, the programmable MAC may determine whether the CRC passed or failed. For example, the CRC value may be compared to a value communicated in a frame header for purposes of validating received data frames.
The CRC engine may advantageously utilize one or more polynomials as determined by the programmable MAC or other system componentry. Further, the CRC engine of a disclosed embodiment may receive data of varying sizes (e.g., 4-byte, 8-byte, 32-byte) such that write processes may be optimized.
The programmable CRC engine thus improves the performance of a cable modem incorporating programmable MAC functionality by moving computationally burdensome functions to hardware while keeping control functions within software. Further, the programmable nature of the CRC engine provides flexibility to support a variety of different MACs and data frame types.